As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
IN CRUISE FLIGHT FL 340 TOWER REPORTED US 400' LOW THEN 600' HIGH WITHIN 3 SECONDS ON XPDR NR 1. SWITCHED TO TRANSPONDER NR 2, CENTER REPORTED US AS STABLE AT FL 340. REMOVED AND REPLACED NR 1 TRANSPONDER PER EO REF 8-3454-7-5001 AND MM REF 34-54-00-5.